I can tell you that my favorite social media site is Twitter. I absolutely love it. If you get on there and you be yourself and just start forming relationships with people, I think you will be pleasantly surprised at the connections that can be made that can be beneficial to your business. It's really a delicate balance of being personal and talking business, I think.
If you get on there and talk only about personal issues, you might not make any connections that will further your business. Or you might make a connection with someone who is perfect for JV'ing with or maybe for hiring you - but, since you have only talked about personal issues - that level of connection is not made with that person.
On the other hand, if you get on there and talk all business, you might bore the ever-lovin' daylights out of people. LOL
So it's an ebb and flow of balancing personal with business.
There are tons and tons and tons of other sites, though, so to fully answer your question, we would have to sit here all day - and my fingers would get super tired from typing all of that out. LOL
There is another answer, though - and that is the great report by Michelle MacPhearson, . It is free and it is incredibly helpful.